Division emphasizes importance of cast iron properties.
Refractory cloth filters were tested in green sand molds with ductile iron to determine the efficiency of the filters in removing inclusions from the molten metal. These filters were able to remove inclusions and dross from the iron by forming a soft sticky coating over the fibrous cloth surface.
Double filtration through a single filter produced a matrix with the least inclusions. The high efficiency of these refractory cloth filters in removing even the small inclusions from the first iron to pass through the filter should provide foundrymen with an alternative to the hard-fired ceramic cellular filters, they said.
Designing with Cast Iron
"Grid Method of Cast Iron Selection for Designed Applications"(90-124) was presented by J.F. Janowak, Grede Foundries, inc. (90-124) Two dimensional grids were developed to illustrate the wide range of cast iron properties. There are three basic characteristics used to form the grid: the metal matrix, the carbon-rich particle and the chemical composition. Each has a significant influence on design performance characteristics.
Designers will need to specify the significant controlling factors by working with foundries to ensure that the desired properties will be present in the iron castings, Janowak said. This method clearly enhances the general understanding of cast irons and serves to aid in selection of the best cast iron specific applications. Through improved understanding of the family of cast irons, their increased use can be fostered.
Research into the "Methods for Ensuring Sound SG iron Castings," (90-001) was carried out by A. Louvo and P. Kalavainen, VTT Metallurgy Laboratory and J.T. Berry and D.M. Stefanescu, Univ of Alabama. The risering and feeding system design for SG iron castings can be rationalized and eased significantly through the use of computers.
This operating system utilized windowing and pulldown menus driven with a mouse. The link and feedback between solidification simulation and empirical designing techniques based on the modulus method introduced in this work should make it possible to supply castings with improved yields without pouring numerous test samples or iteration rounds in computer simulation. The simulation gives method engineers more accurate data on the entire heat transfer process and solidification sequence in the case of ductile iron.
In "Heat Transfer-Solidification Kinetics Modeling of Structural Transitions: Chill Formation in Gray Iron," (90-156) by K.G. Upadhya, D.K. Banerjee, D.M. Stefanescu and J.L. Hill, Univ of Alabama, noted that the unexpected occurrence of chill is a major cause for scrapped gray iron castings with varying section sizes. Its prediction and control can therefore reduce scrap costs. A physical model for the evaluation of chill formation in iron castings was developed, based on the concept of a critical cooling rate at which a gray to white transition occurs.
It was shown that this model can describe the experimental map of structural transition over the whole section of the casting. Agreement was achieved between the experimental and calculated cooling curve data and calculated chill depths at selected locations in the casting. This type of model is expected to be beneficial for foundries in which the melting and inoculation processes are accurately monitored. it will predict the structural map for any given casting provided that certain data is incorporated into the model.
"The ability to predict matrix microstructure in ductile iron is an important factor in controlling the quality of the castings," said D. Venugopalan, Univ of Wisconsin/Milwaukee. The factors which determine the matrix microstructure are within the foundry's control or they can be determined by existing methods. This work (90-121) was aimed at developing a viable model for microstructure development in ductile iron with ferritic, pearlitic or mixed matrices.
It was applied to calculate continuous cooling transformation diagrams for some unalloyed and alloyed ductile irons. The model was capable of accurately predicting the transformation kinetics and matrix microstructure in ductile iron. When combined with models for cooling conditions and mechanical properties, it can form the basis of a system for predicting ductile iron properties in castings.
C. Fung, P. Bartelt, F. Bradley and R. Heine, Univ of Wisconsin/Madison described operating practices at each of several participating foundries to determine a statistical model for predicting shrinkage in ductile iron castings (90-71). Processing and thermal analysis variables affecting shrinkage were identified. Additional statistically designed plant experiments are being conducted to obtain data for the development of a more reliable shrinkage prediction model.
Austempered Ductile iron
Outstanding mechanical properties obtained from ADI are well known and a number of investigators have studied the transformation and resultant microstructures obtained from this treatment. The wide range of properties offered by ADI has attracted worldwide attention and many development projects are underway with the objective of opening up new markets. A more thorough understanding of the effects of heat treatment variables and alloy content on the properties of ADI has to be more firmly established. Two papers were presented that reflect this attitude.
In the first (90-142), "A Study of the Austempering of a Ni-Cu Alloyed Ductile Iron," T.S. Shih, P.Y. Lin and C.H. Chang, National Central Univ, and C.R. Loper Jr., Univ of Wisconsin/ Madison, investigated tensile properties, hardness and impact energy to determine a window of operation that would yield optimum properties in austempered ductile iron. The mechanical properties of an ADI alloyed with 0.9% Ni and 0.6% Cu demonstrated that a processing window can be established.
At higher austenitizing temperatures the toughness and ductility decreased due to carbide precipitation. Fractographic analysis exhibited reduced ductility where martensite was present and where carbide precipitation occurred. Crack direction was determined by the microstructural constituents least able to strain or deform.
M. Grech, Univ of Malta, and J.M. Young, Univ of Birmingham, examined mechanical properties after a 1.6% Cu and 1.6% alloyed ductile iron was austenitized at 900C for two hours and austempered in the range of 240-400C for four hours. Results show that this alloy combination offers an excellent combination of high ductility and moderate strength, making this alloy a suitable candidate for high toughness service conditions. (90-160)
Heavy Section Ductile iron
It is well recognized that the thermal center of heavy section ductile iron castings is susceptible to the formation of a number of types of deteriorated graphite, seriously impairing the mechanical properties. Previous studies have investigated the mechanism of deteriorated graphite formation and the need to establish procedures to prevent its occurrence.
Certain forms of deteriorated graphite have been related to the occurrence of deleterious elements in the melt (i.e., Bi, Sb, Pb, etc) primarily due to raw materials, but not ascribed to the fade of the nodulizing element, Mg. It has been recommended that controlled levels of the rare earths be added to the melt to neutralize the effect of these elements. The role of these elements and the use of rare earths in thin and thick casting sections reckons further investigation in order to improve the quality of ductile iron castings.
A study to re-evaluate the role of Sb and/or rare earths on the graphite morphology and mechanical properties of heavy section ductile irons (90-163), was conducted by B.C. Liu, T.X. Li and Z.J. Rue, Tsinghau Univ, X.Y. Yang and E.Q. Huo, First Heavy Machinery Plant, and C.R. Loper, Jr., Univ of Wisconsin /Madison. Auger microprobe analysis revealed that Sb scavenges oxygen absorption at the graphite/melt interface during solidification. Sb also absorbs onto the graphite surface during solidification resulting in improved graphite morphology. Three heavy section castings were produced by a commercial foundry to verify results obtained using a thermal simulation furnace. Combined treatment of ductile iron with Sb and rare earths was found to result in high quality, heavy section ductile iron.
"Production and Evaluation of Heavy Section Ductile Cast Iron,"(90-43) by H. Itofuji, K. Kawamura, N. Hashimoto and H. Yamada, UBE Steel Co, Ltd, reported on results of a study evaluating a 36 ton ferritic ductile casting. The casting design was simulated by computer and the results agreed with practice. The most important process parameter was found to be controlling the graphite morphology, such as nodule spacing, size, etc.
H. Itofuji and H. Uchikawa, UBE Steel Co, Ltd. investigated substructure and element segregation using an SEM, TEM, EPMA with a colored mapping system, optical microscopy, etc (90-42). The chunky graphite formation mechanism was considered under the site theory, which was newly proposed to explain the graphite formation mechanism in liquid and solid cast iron. The main cause for the chunky graphite formation was considered to be a lack of Mg gas bubbles as the free surface in the melt, Mg segregation as the metallic element and inclusions at the old austenite-residual melt interface.
Mechanical Properties of Commercial iron
Other presentations covered some of the variables and production conditions that are present in commercial foundries and their influence on mechanical properties.
J.F. Janowak and A. Alagarsamy, Grede Foundries, Inc, and D. Venugopalan, Univ of Wiscons in/Milwaukee, addressed some of the conditions that influence the fatigue properties of commercially produced ductile iron (90-123). Increased fatigue endurance limits (FEL) resulted from increased composite matrix microhardness, pearlite content, cleanliness, soundness, nodule count and optimized chemical composition. An iron metallurgically designed for high fatigue strength provided the highest fatigue strength in the as-cast as well as the austempered condition by using low Mn, low residuals, alloying with Ni, Cu, and Mo, and filtering. Care in heat treating was advised. It was found that production ductile irons have higher FELs than those produced in a laboratory.
In a study to determine the "Effects of Alloy Additions on the Microstructure and Mechanical Properties of Commercial Ductile Iron," (90-122) D. Venugopalan, Univ of Wisconsin/ Milwaukee and A. Alagarsamy, Grede Foundries, Inc, analyzed 15 commercially produced ductile irons with varying levels of Si, Mn, Cu, Ni, Mo and P. The properties were studied in the as-cast, normalized and annealed conditions. Tensile and yield strengths, and elongation correlate linearly to the composite matrix microhardness. The influence of alloying elements on the ferrite and pearlite contents of the matrix microstructure were expressed in terms of regression equations, which were derived from the experimental data to form the basis of a self-consistent model for predicting the mechanical properties of ductile iron.
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|Date:||Jun 1, 1990|
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